Devices that directly convert chemical energy into electric energy are known alternatively as electrochemical cells, galvanic cells, or batteries. These cells, referred to herein simply as cells, consist of a positive electrode and a negative electrode immersed in, or in contact with, an electrically-conductive medium called the electrolyte. The electrodes are generally separated by a porous diaphragm, also called a separator, which is ususally made of an electrically-insulating material.
The cell diaphragms should be thin, yet must have good mechanical strength in order to withstand handling during manufacture and use. In the past, paper, synthetic polymer resin impregnated paper, textile fiber cloth, non-woven cloth, and the like have been used as diaphragm materials. More recently, porous membranes of polypropylene are widely used as a diaphragm material. The presence of electrolyte in the pores of the diaphragm provides a path for ionic transfer of electrons, from one electrode to the other. This chemical transfer of energy occurs when the external circuit to which the cell is connected is closed, and the cell is called on to provide electricity.
In high energy density cells, in particular those in which lithium metal or lithium compounds are used, continuous high-rate or short-circuit discharges can lead to serious safety problems. The excessive heat generated by the high-rate discharge can create extremely high internal pressures and temperatures in the cell which may result in an explosion, or melting and ignition of lithium which may result in a fire. Such problems have been known to occur in, for example, cylindrical lithium cells for use in cameras. These cells consist of a thin porous diaphragm inserted between positive and negative electrodes which are spirally wound so that substantial electrode surface area capable of high energy output is created in the small volume of the device.
To prevent such occurrences, diaphragms having a shut-down function intended to interrupt the chemical reaction in the cell and prevent generation of excessive heat during high-rate discharge have been developed. The porous diaphragm melts and flows at a temperature lower than the melt point of lithium (about 180.degree. C.), thus causing the pores to be blocked, and shutting down the chemical reaction generating the heat. Polypropylene diaphragms are used for shut-downs desired at about 140.degree. C. and polyethylene diaphragms used for shut-downs desired at about 120.degree. C.
A problem with such diaphragms, however, is that when too much heating occurs the diaphragm melts and flows excessively, and sags or tears away from between the electrodes. Thus, a short circuit across the gaps in the diaphragm can be established between the electrodes, leading to generation of additional heat and eventual outbreak of fire.